Support some non-structural (nominal) type matching

Proposal: support non-structural typing (e.g. new user-defined base-types, or some form of basic nominal typing). This allows programmer to have more refined types supporting frequently used idioms such as:

1. Indexes that come from different tables. Because all indexes are strings (or numbers), it's easy to use the an index variable (intended for one table) with another index variable intended for a different table. Because indexes are the same type, no error is given. If we have abstract index classes this would be fixed.

2. Certain classes of functions (e.g. callbacks) can be important to be distinguished even though they have the same type. e.g. "() => void" often captures a side-effect producing function. Sometimes you want to control which ones are put into an event handler. Currently there's no way to type-check them.

3. Consider having 2 different interfaces that have different optional parameters but the same required one. In typescript you will not get a compiler error when you provide one but need the other. Sometimes this is ok, but very often this is very not ok and you would love to have a compiler error rather than be confused at run-time.

Proposal (with all type-Error-lines removed!):

// Define FooTable and FooIndex
nominal FooIndex = string;  // Proposed new kind of nominal declaration.
interface FooTable {
  [i: FooIndex]: { foo: number };
}
let s1: FooIndex;
let t1: FooTable;

// Define BarTable and BarIndex
nominal BarIndex = string; // Proposed new kind of nominal declaration.
interface BarTable {
  [i: BarIndex]: { bar: string };
}
let s2: BarIndex;
let t2: BarTable;

// For assignment from base-types and basic structures: no type-overloading is needed.
s1 = 'foo1';
t1 = {};
t1[s1] = { foo: 1 };

s2 = 'bar1';
t2 = { 'bar1': { bar: 'barbar' }};

console.log(s2 = s1); // Proposed to be type error.
console.log(s2 == s1); // Proposed to be type error.
console.log(s2 === s1); // Proposed to be type error.

t1[s2].foo = 100; // Gives a runtime error. Proposed to be type error.
t1[s1].foo = 100;

function BadFooTest(t: FooTable) {
  if (s2 in t) {  // Proposed to be type error.
    console.log('cool');
    console.log(t[s2].foo); // Proposed to be type error.
  }
}

function GoodBarTest(t: BarTable) {
  if (s2 in t) {
    console.log('cool');
    console.log(t[s2].bar);
  }
}

BadFooTest(t1); // Gives runtime error;
BadFooTest(t2); // No runtime error, Proposed to be type error.
GoodBarTest(t1); // Gives runtime error; Proposed to be type error.
GoodBarTest(t2);

未指定任何文件、测试或入口点。首先审查提议的名义声明以及 index、callback 和 interface 的使用场景；要视为完成，需要就设计达成一致，并明确对所列无效赋值和调用的类型检查行为。